1. Field of the Invention
The present invention relates to an optical amplifier for use with a high-power, solid state laser, and more particularly to an optical amplifier which includes two elongated slabs of a solid state lasing material, for example, a yttrium-aluminum-garnet (YAG) host crystal that is doped with a rare earth metal ion, such as Erbium, Holmium, Neodymium, and Thulium or other rare earth metal ion, wherein the elongated slabs are configured to compensate for astigmatism which results in optical phase distortion of the laser beam wavefront due to thermal gradients in the traverse direction of the slabs.
2. Description of the Prior Art
Solid state lasers are known to include an optical amplifier, which includes a generally rectangular elongated slab of lasing material, such as a rare earth doped yttrium-aluminum-garnet (YAG) crystal, defining four lateral surfaces and opposing end faces. The refractive index of the slab is generally high while the index of refraction of any cooling medium disposed adjacent the lateral surfaces of the slab is selected to be relatively low. This difference in the indices of refraction at the slab to cooling medium interface causes a light beam entering an end face of the slab to be totally internally reflected within the slab. During such a condition, light entering one end face of the slab is reflected in the slab in a zig-zag pattern. As such, such optical amplifiers have become known as zig-zag amplifiers. Examples of such zig-zag amplifiers are disclosed in U.S. Pat. Nos. 3,679,999; 4,730,324; 4,852,109; 5,305,345; 5,646,773; 5,555,254; and 5,307,430.
The slab of lasing material is pumped by an external light source. The light source is used to pump the atoms in the lasing material to a relatively high energy metastable state. Various light sources are known for this application. For example, diode arrays are known to be used in such application. Examples of solid state lasers which utilize diode arrays for pumping are disclosed in U.S. Pat. Nos. 4,852,109; 4,949,346; 4,984,246; 5,271,031; 5,305,345; 5,317,585; and 5,351,251.
The pumping of atoms within the slab produces considerable heat therewithin. Since the slab is cooled on its surface, thermal gradients in a transverse or vertical direction (i.e. along a minor axis) occur. (As used herein the axis in the zig-zag or horizontal direction relative to a slab having a cross section which is generally square or rectangular is referred to as a major axis. The minor axis is generally perpendicular to the horizontal axis). These thermal gradients result from the temperature difference between the hot inner portions of the slab with respect to the cooled lateral faces. The thermal gradients are not a problem along a major axis of the slab since the zig-zag beam path within the slab causes the beam to average the thermal gradients in that direction. However, the thermal gradients are not averaged along the minor axis resulting in an astigmatism. As discussed in U.S. Pat. No. 4,730,324, the astigmatism results from the thermal gradients which are known to cause a deformation of the end face and lateral faces as well as variations in the refractive index in the slab resulting in a positive lensing affect along the minor axis and a slight negative lensing affect along the major axis causing optical phase distortion of the wavefront of the output laser beam.
Various methods are known for compensating for thermal gradients along the minor axis of the slab, for example, as disclosed in U.S. Pat. Nos. 4,852,109 and 5,646,773. In both of these patents, cooling channels are disposed along the lateral faces of the slab. The volumetric flow rate and the temperature of the fluid within the cooling channel may be varied in order to enable the temperature of lateral faces to be adjusted.
Although the systems disclosed in the '109 and '773 patents are able to provide compensation for the affects of the thermal gradients in the slab, such solutions for compensating for the positive lensing affect are relatively complex. As such, there is a need to develop a system with reduced complexity for reducing the astigmatism along the minor axis of the slab due to temperature gradients.